As gas-barrier resins not containing chlorine, there are known polyamide resins such as nylon 6, polymetaxylyleneadipamide (hereinafter referred to “N-MXD6”), etc., and ethylene-vinyl copolymers.
Of those, N-MXD6 has good mechanical properties and is excellent in oxygen-barrier performance, especially in oxygen-barrier performance in high-humidity environments, and is therefore favorable as a food-wrapping/packaging material that is required to have oxygen-barrier performance after thermal sterilization treatment such as boiling or retort treatment.
As food-wrapping/packaging materials, used are multilayer bottles, blend bottles and stretched films using a polyethylene terephthalate and a metaxylyleneadipamide. Also used are laminate films formed by laminating a base film of a polyolefin or the like and a resin film containing a polyethylene terephthalate and a metaxylyleneadipamide, and mixed resin films formed by further mixing a nylon 6 or the like and molding the resultant mixture.
As food-wrapping/packaging materials, plastic materials are being used these days in place of glass, and the plastic materials are being required to have further improved barrier performance against oxygen, carbon dioxide, water vapor, etc. Depending on the use thereof, food-wrapping/packaging materials may be required to be excellent in transparency and moldability.
On the other hand, heretofore, resin compositions and resin molded articles containing nylon 6, N-MXD6 or the like have been improved variously. For example, PTL 1 discloses a film and a sheet obtained from a polyamide resin composition in which N-MXD6 is mixed with any other specific polyamide having a high crystallization rate such as nylon 6 or the like. PTL 1 says that the film and the sheet are flexible and can keep excellent transparency even in high-humidity atmospheres.
However, the film and the sheet described in PTL 1 are molded from a resin composition containing any other polyamide mixed therein, and therefore have a problem in that the gas barrier performance thereof is poor, as compared with a film of N-MXD6 alone.
NPL 1 discloses a composite material in which a clay mineral having a molecular size is dispersed in nylon 6, and the nylon 6 and the clay mineral bond with ion-bonding, that is, a so-called “nylon 6-clay hybrid”. NPL 1 says that, in the nylon 6-clay hybrid, the growth of the spherocrystals is retarded by the clay layer and the size of the spherocrystals is controlled to be not more than the wavelength of the visible light, and therefore the visible light transmittance of the hybrid is increased compared with that of ordinary polyamide.
N-MXD6, which is a crystalline polyamide as well, could be expected to have the same effect, but for expressing the effect, 1% of a clay mineral must be added thereto.
However, with respect to a film, a sheet and the like molded from a composite material that contains N-MXD6 and, as added thereto, at least 1% of a clay mineral, the mechanical properties such as impact resistance lower, and the color tone worsens.
In a case where a film formed of a composite material that is prepared by blending N-MXD6 with an inorganic substance such as talc, mica or the like is used for boiling treatment and retort treatment, the film can be expected to have whitening resistance after heating. However, in the composite material of the type, the crystallization rate is accelerated by 2 times or more as compared with that in additive-free N-MXD6, and therefore, in a case where the material is used as a molding material for stretched films or deep-drawn cups and the like to be formed of a sheet, the crystallization rate in the material is too high and therefore the films and the sheets could not be stretched owing to crystallization therein, and, as a result, there occur some problems in that the films and the sheets would be broken or would be stretched unevenly, that is, the moldability thereof is extremely poor.
PTL 2 discloses a film, a sheet and a hollow container formed of a polyamide resin composition that is prepared by adding, to a polyamide MXD 6, a specific amount of a diamide compound or a diester compound obtained from a specific fatty acid such as ethylenebis-stearylamide or the like and a specific diamine or a specific diol. The reference says that the film and the like whiten little and can therefore keep transparency during storage in a high-humidity atmosphere or on being kept in contact with water, especially with boiling water, in an amorphous and non-stretched state or even in an amorphous and low-stretched state. However, in PTL 2, nothing is investigated relating to the barrier performance against oxygen or the like of these sheets and others.
PTL 3 describes an organic nucleating agent for polypropylene that contains an alkyl-substituted aromatic aldehyde and a bis(dibenzylidene)sorbitol acetal to be obtained from the relevant acetal.
As shown in PTL 2, it is known that the organic nucleating agent for polypropylene dissolves in polypropylene and constitutes a nanometer-level network structure, and starting from the network as nuclei, polypropylene spherocrystals of nanometer-order grow. The spherocrystals have a nano-level crystal size, and are therefore excellent in transparency, and the nucleating agent is widely used for improving the transparency of polypropylene.
However, in a case where the bis(dibenzylidene)sorbitol acetal is added to a polyamide, there still occurs a problem that the barrier performance of the resultant mixture greatly worsens in high-humidity environments, though the additive exhibits an effect for improving the transparency of the composition of polycondensed resin such as a polyamide or the like and an effective for improving the barrier performance at a low humidity.